The following relates to the non-destructive examination arts, component maintenance arts, and related arts.
Heat exchangers, such as steam generators, are commonly used in electric power generation. A typical arrangement of a power plant includes a nuclear, fossil fuel-fired boiler, or other water boiler system that heats water to a boiling, sub-cooled, or other heated state. The output is a mixed-phase, two-component water/steam mixture that is fed into a steam separator where dry steam is separated from the mixture and used to drive a turbine or to perform other useful work. In a variant approach, a steam generator receives a saturated liquid and also receives secondary coolant in the form of liquid water, and heat transfer in the steam generator results in the boiling of the secondary coolant to produce the steam while maintaining fluid isolation between the saturated liquid (that is, primary flow) and the secondary coolant. This latter arrangement is beneficial in systems such as pressurized water (nuclear) reactors (PWR) in which the reactor may impart radioactivity on the primary coolant.
In such steam generators, the quality of the steam is an important consideration. High quality steam is desirable as it contains little or (ideally) no liquid water. Liquid water in steam can lead to moisture-induced degradation of components, including, for example, turbine components, that are exposed to such steam.
Various technologies can be employed to perform steam separation, including centrifugal separators, scrubbers and chevrons. Alternatively, steam separation can be negated via the use of once-through steam generators. The secondary side fluid exits once-through steam generators in a super-heated state, thus removing the need for steam separation.
In cyclone or centrifugal steam separator components, high-speed rotation is imparted into the fluid flow so as to separate steam and water by centrifugal force. Cyclonic separators are well-suited for use as a second stage or drying phase in the steam generator, where high flow rates of (mostly) steam facilitate efficient centrifugal separation. Cyclonic steam/water separators (also called moisture separators, steam separators, or similar nomenclature) can be active devices, for example, using a rotating turbine to impart rotational flow, or can be passive components in which fixed vanes are oriented to impart rotation to an existing high-velocity steam flow. Surfaces of the cyclonic separator are configured to collect moisture from the rotational flow while allowing the dried steam to pass. Passive secondary cyclonic steam separators are commonly used to improve steam quality in steam generators.
Because cyclonic steam separators are exposed to moisture during normal operation, the potential exists for moisture-induced surface degradation. The cyclonic steam separator components may be visually inspected during steam generator maintenance outages, sometimes including photographic recordation of surface condition. The inspection can be hampered by time constraints and is also usually coordinated with other concurrent maintenance operations introducing further timing and scheduling constraints.
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